Replace gtk_debug_flags with getter and setter functions

Preferrably should be made just into a local variable for libgtk like
_gdk_debug_flags for libgdk. But for now used by
gtk/tests/textbuffer.c and modules/printbackends/cups/gtkprintbackendcups.c.

gtk/gtkprintjob.c

@@ -485,7 +485,7 @@ gtk_print_job_get_surface (GtkPrintJob  *job,
   
 #ifdef G_ENABLE_DEBUG 
   /* If we are debugging printing don't delete the tmp files */
-  if (!(gtk_debug_flags & GTK_DEBUG_PRINTING))
+  if (!(gtk_get_debug_flags () & GTK_DEBUG_PRINTING))
 #endif /* G_ENABLE_DEBUG */
   g_unlink (filename);
   g_free (filename);